Yu would have to know the recovery capacity of both the old and new units. Then if the sum of the old one's recovery plus 80 gallons is equal, or close to the sum of the new unit's recovery plus 40 gallons, and the new one's recovery is higher than the old one, then you are okay, otherwise your capacity has been diminished.

: We're in the process of closing on a house and the Seller was supposed to repair a leaking 80 gallon hot water tank. He replaced it with a "new", smaller (~40 Gallon) GlowCore tank. Did I get short changed?